I'm not against buffers but against the general UI flow, which requires either mouse clicks or hitting `C-x o'.
For me the crucial difference between mini-buffer completion and pop is that one does not alter the layout of my windows while the other does. Have a helpful list of suggestions to the right while my cursor remains undisturbed is very intuitive. By contrast having a new window pop up moves _my_ focus but fails to move emacs' focus. I would be very happy if, having configured emacs to pop a completion buffer emacs' focus moved there immediately.